Capacitor end seal process



June 14, 1960 A. L. scHlLs CAPACITOR END SEAL PRocEss Filed Feb. 13, 1956 f f, l

l 'lllllllllll llllllllllllllh FIG.1

,MIS

FIG.2

CAPACITOR END SEALPROCESS Alvin L. Schils, Nashua, vN11., assignor vto Sprague. Eleotric Company, North Adams, Mass., a corporation of Massachusetts Filed Fears, 1956, ser. No. 564,989

2 claims. (cl. isi-59) The present invention relates to a new and improved process for producing resin bonded oating disc end seal constructions.

For many years small electrostatic capacitors have been produced utilizing external enclosures such as metal cans to protect the capacitance section. The metal enclosures are of a hermetically sealed construction utilizing numerous types of end seals known within the art. A particularly desirable end seal incorporating a resin bonded tloating `disc construction is set forth in the Walter C. Lamphier United States patent application, Serial No. 488,329, tiled February 15, 1955. Although the product disclosed by Lamphier is remarkable yfor its simplicity and ability to produce an eifective seal whichcan be cycled from extremely low to extremely high temperatures without :failure of the seal, the disclosed process is time consuming and difficult to control quality-wise.

It is an object of the present invention to produce a capacitor end seal using a resin bonded floating disc construction by a process of markedly improved efficiency and quality. A further object is to produce an easily assembled and cured resin end seal.

Still further objects of the invention, as well as the advantages thereof, will be apparent from the body of this specification, the appended claims, and the accompanying drawing in which: Fig. 1 is a sectional view of a capacitor assembly including the resin bonded tloating disc end. seal constructed according` to this invention; and Fig. 2 is an expanded fragmentary sectional view of the components of .the resin bonded floating disc end seal in position in ,the mouth of acapacitor prior to the final processing steps of this invention.

`In its broadest scope this invention is concerned with an improvement in the process of producing encased electrical components having a resinous end seal which improvement includes the steps of pelleting amineral filled epoxy resin annular disc before placingit in the end of the casing'tobesealed and thereafter thermally..4 curing the end seal assembly simultaneous with vibration of said assembly.

A representative finished product resulting from the process of this invention is illustrated in Fig. l wherein there isshown a capacitor assembly in which there is employed a cylindrical metallic container .11, housing' a wound capacitor section 19, and having an open end '12, throughfwhich Aterminal lead 13 of the capacitor projects. About the lead is seen a filled epoxy resin mass in, which is positioned a ceramic washer 14. If desired, a second washer 28 of 'tibrous material such as paper can be used to keep 'the epoxy resin from contact with the capacitor section 19 and dielectric impregnant 16, thereby avoiding any inneraction between them. 'Ihe can has a bottom vhole 29 through which Ianother terminal lead 20 ofthe enclosed capacitor projects. The hole 29 is closedby means of lead-tin solder 21, or other suitable` metal alloy, to hermetically seal the unit. Before sealing, the unit can be impregnated with a liquid dielectric 16 that enters through hole 29 and tills all pores and voids within capacitor section 19, as well as the space between the container 11 and the capacitor.

The preferred resin is of the epoxy type which is wellknown in the art, some of them being disclosed in theA Buck etI al. United States Patent No. 2,569,929, the Wiles Patent No. 2,528,934, the Greenlee Patent No. 2,542,664,

the Bixler Patent No. 2,512,996, the Bender et al. Patent4 No. 2,506,486, .the Greenlee Patents No. 2,510,885 and 2,510,886, the Newey et al. Patent No. 2,553,718, the E. S. Narracott Yarticle Application of Some Epoxide Resins in the Plastics Industry in British Plastics, October l, pages 341-345 and the W. J. Marmion article Epoxide Resins in Research, September 1954, pages 35i-355. In use as an end seal the resin normally is filled with a material which modifies the temperature coeliicient of expansion so as to approximate that of thev metal housing of the unit. Suitable illers include alumina, silica, mica, and other similar materials which are admixed into the resin in an amount dictated by the desired temperature coetlicient of expansion. Y

Generally in the practice of the process of the invention the following steps are performed, although it is to be understood :that in practice several of the steps can be combined into a single operation: blending of the resin and ller into a uniform mixture; pelleting the filled resin into Ian annular disc, `assembly of said pelleted disc and an annular ceramic washer into the open end of the capacitor casing (as shown in Fig. 2), and heating of the assembled unit simultaneous with vibration of said assembly.v

In the blending of the resin and .the filler, it is desirable to substantially uniformly distribute the particles of fliller into the resinous mass. Such a substantially unistorm distribution avoids uneven distribution of mechanical stress in the nished end seal when the hermetically l sealed unit is temperature cycled over an extreme temperature range, eg., from v 50" C. to +125 C. Typical of a composition which is blended for end seals of metal cased units is 30 parts by weight of the condensation pronduct of 4 mols of diphenylopropone with 5 mols of epichlorohydrin in the presence of an excess of 10% aquei od of about l5 to about 20 minutes. Generally the" of rela-- blending should be carried out in an atmosphere tively low humidity, eg., less than 10%.

The blended powder which has been stored in a pelletted into the deture-free atmosphere is thereafter sired annular disc dimension.

The preforrning of these resin discs can be held to a thickness variation of approximately il mil by use of a conventional press using punch die and core rod, which presses are well known to the art. Useful pressures for` obtaining'the preforms according to the inventive process', range from about 1500 to 3000 pounds per square inch" for the filled epoxy resin composition set forth above;

Now that the annular disc preform has been produced,

the product can be distributed to the assemblyoperators for incorporation into the capacitor in the manner shown in Fig. 2. After assembly of `the capacitor section into'V the metal container, a preform annular pellet andA thereafter a steatite or similar ceramic disc which may be"- either slotted or annular is placed over the assembly and racked up into suitable processing jigs, which jigs may include for example 1000 piece lots. In contrast to the w previous procedure, it is to be noted that the ceramic annular disc merely sits on the surface of the resin disc.

The pelleted annular disc is flowed onto the inner sur- Paten-ted. June 14, 1960` parts by weightA Vamplitude of about 2 inches.

i further thermal treatment.

c 1 f entonar face of the container and' onto the'lead Wire under thermal and vibratory influences. The racked metal can type of units set forth immediately above are exposed to an atmosphere of elevated temperature for an initial precure toboth dispose the resinV into Contact with both the inner surface of the metal casing and the terminal-lead wire, and to,Y allow the immersion of the ceramic annular disc into the resin Vdiscuntil, at least the outer surface of the annularv ceramic disc is substantial planar to the surfaceV ofthe resin.- Such precure is accomplished forthe eral-filled" epoxy resin by heating for about 20 minutes inan atmosphere of about 275 Simultaneous with thisn exposure to temperature, the units are vibrated so as to'facilitate the thermal iiow of the resin. Surprise ingly enoughit has been found that low level vibration improvedquality and mechanical nature. A convenientV way of carrying out the precure of the units is to place them in a conveyor oven with the desired temperature and utilize the vibration of the conveyor belt as it passes through the oven to seat the ceramic annular disc in the aforementioned uniform manner. The preferred vibration has a frequency of about 15 cycles per second and Broadly speaking the frequency can range upwards from 5 cycles to the ultrasonic range,l egt, 5000 to 6000 c.p.s. and the amplitude to be within Athe invention should not exceed inches- Finally the metal encased units are subjected to a This is obtained for the silica filled epoxytype of end seal construction by exposure to a'tem'perature of from' about 250 F. to about 350 F.

Yingv its scope, a capacitor section made of convolutely wound ribbons of aluminum foil and' paper, with the respective electrode foils projecting from opposite ends of the Winding, had,0.02 in diameter lead wires of diptinned. copper separately soldered to their respectively extensions. lThis sectiony was placed in an open ended cylindrical brasscan. having an outerdiamcter of 0.17 Si". The-closed end of the container had a central opening 0.041" in diameter, through which opening, one of the lead, wires was passed. The end seal composition was made up of 60 parts by weight of powdered epoxy resin ai1d,30,.parts by weight of powdered silica passingjthrough a, 3,00 mesh screen,` and 2 parts by weightof dimethylene triamine, the catalytic agent The mixture was blended for a period of minutes inl a blending machine. The powder was fed into a Stokes Model F presswhich produceda pellet of 0.116" thickness, an outer diameter. of 0.146" and a slot extending to the center of the pellet 0.50, wide. A paper washer was placed over the central lead wire and disposed against the exposedv end of the capacitor section. The preform pellet was placed down against the paper spacer and thereafter a steatite annular disc having anY outer diameter of 0.13 and an inner diameterv of 0.05 and a maximum thickness of 0.078" was slipped vover the lead protruding from the open end and disposed against the preformed resinous disc. 'Ihe unit Wasfthen placed. in a rack and sent through a conveyor belt oven exposing it for minutes to a temperature of approximately 275 F. During this exposure continual vibration` of the said section was developed by the movement-of the conveyor belt which vibration had an ampliv tude of 2 inches lat a frequency of about 15 cycles per second. Thereafter theunit was removed from the convveyorv belt and placed into an oven at 300 F. for two hours, obtaining a full and complete .cure of the end 4 seal construction. Finally the unit was impregnated with polyisobutylene by vacuum 'techniques and thel impregnation channel was sealed by solder. The resulting assembly can satisfactorily withstand as many as 100 thermal cycles of cooling to 55 C. and heating to 100 C. without apparent change, in fact the unit can'be taken directly from a dry-ice bath, held at -55V C. and immediately dropped into boiling water `without damaging the seal or any other portion of the assembly.

The advantages of our invention are in the most part obvious. The reject levelis reduced by 95% .over that resulting with prior art techniques. The Vsurprising improvement in quality and uniformity is simultaneous with .substantial vsavingfin the amount of resin used, thus markedly reducingthe cost of the unit as well as reduc-` ing thel amount of labor required'for production.

As many apparently widely diiferent embodiments of this invention may be made without 'departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments herewire ofsaid capacitor, said process comprising blending 4 mineral filler and epoxy resin particles, pelleting said blended filler and resin into a preform adaptedY to approximatelyll the openend of said casing and provided with passage means to accommodate said lead-wire, positioningsaid preform in said open end with said leadwire passing therethrough, superposing ra ceramic annulus on said preform around said lead-wire, heating and vibrating the assemblage of casing, lead-wire, preform, and annulus whereby said annulus is seatedin said preform substantially flush therewith, and said preform ilows into intimate engagement with saidV casing and said lead-Wire while substantially filling said open end of said casing.

2. A process for providing an end seal for an open end of, a metallic capacitor casing wherein said seal has Va ceramic annulus embedded in a mineral-filled epoxy resinous mass contiguous with said casing'and a leadwire of said capacitor, saidprocess comprising blending.

mineral filler and epoxy resin particles, pelleting said blendediiller and resin into a preform adapted to approximately ll the open end of said casing andprovided with passage means to accommodate saidl lead-wire, positioning said preform in said open end vwith said lead-wire passing therethrough, superposing a ceramic annulus on said preform around. said lead-wire, precuring said preform by exposure to an atmosphererof about 275 Fl forl about 20 minutes while vibrating said casing, and thereafter heating at a temperature of'frorn about 250' F. to about 350 FL for a period of from about 1% to about 21/2 hours, whereby said annulus is seated in said preform substantially ush therewith, and said preform flows into intimate engagement with said casing and said lead-Wire while substantially filling saidopen end of said casing. Y

References Cited'Y in the rile of this patent 

